Net undercount is the difference between the PES estimate of the number of people who should have been counted in the Census and the actual Census count. The Census count includes persons who have been imputed in non-responding dwellings in the Census, that is, the Census count equals persons counted on Census forms plus imputed persons for non-responding Census dwellings.

Net undercount is therefore a measure of the combined outcome of Census enumeration and data processing. For more information about imputed persons for Census non-responding dwellings and the adjustments made for them in the PES estimates, see Components of net undercount and the Identifying Census late returns Technical Note (in Explanatory Notes).

2011 PES net undercount estimates

In the following tables, net undercount is presented as both level estimates of persons and rates, together with their associated standard errors (SEs). The net undercount rate expresses the net undercount (i.e. undercount minus overcount) as a percentage of the PES estimate of a given population (i.e. as a percentage of the number of people who should have been counted in the Census). All estimates of net undercount based on geography have been calculated on a place of usual residence basis, meaning they are based on the location where a person lived, or intended to live, for six months or more in 2011.

AUSTRALIA

The 2011 Census counted 21,504,721 usual residents of Australia (including imputed persons in non-responding dwellings). This was around 374,540 persons fewer than the estimated population usual residents who were present in Australia on Census night. This equates to a net undercount rate of 1.7%. In other words, 98.3% of the usually resident population were included in 2011 Census counts.

While the net undercount rate decreased from 2.7% in 2006 to 1.7% in 2011, it is important to note that PES estimates of net undercount are not strictly comparable over time due to changes in both Census and PES methodologies. The PES is designed to provide the best measure of Census coverage at a single point in time rather than as a time series, with improvements made to the PES and Census in each cycle. This is particularly true for 2011 with the introduction of Automated Data Linking (ADL) in the PES which has made it difficult to directly compare level estimates and proportions of net undercount from one Census to another. Compositional analysis is therefore much more illustrative.

The ABS has estimated that the introduction of ADL for 2011 has resulted in a net undercount that was 246,985 persons less than if the 2006 methodology had been used. This estimate is subject to sample error. For further details see the Statistical Impact of ADL Technical Note (in Explanatory Notes).

Table 1 shows the net undercount rates and associated standard errors (SEs) for Australia for each Census from 1971 to 2011. The 2011 estimate of 1.7% continues the historical trend of almost complete coverage in Australian Census counts.

1 NET UNDERCOUNT RATE, Australia - 1971-2011

1971

1976

1981

1986

1991

1996

2001

2006(a)

2011(b)

%

%

%

%

%

%

%

%

%

Net Undercount

1.4

2.7

1.9

1.9

1.8

1.6

1.8

2.7

1.7

Standard Error (SE)

0.1

0.04

0.1

0.1

0.1

0.1

0.1

0.2

0.2

(a) Care should be taken when comparing estimates from 2006 onwards with previous years due to changes made to PES estimation and the inclusion of remote areas and discrete Indigenous communities in the PES sample from 2006.

(b) Care should be taken when comparing 2011 estimates with previous years due to changes made in PES linking and matching methodology. For more information see Linking and matching.

AGE AND SEX

The likelihood of counting a person in the Census has traditionally varied according to age and sex. As has been observed in previous Censuses (both in Australia and overseas), young adults are the age group who are most likely to be missed in the Census, with young adult males being more likely to be missed than their female counterparts. In contrast, older adults are much more likely to be counted.

Tables 2 and 3, and graph 4, show that this was also true for the 2011 Australian Census. In particular, males aged 20-24 years again had the highest net undercount rate (7.8%) followed by males aged 25-29 years (7.5%). The net undercount rate for females was also highest for those aged 20-24 years (6.0%). While the undercount rate for 25-29 year old females (4.0%) was higher than most age groups, in contrast to 2006 it was noticeably lower than the rate for females in their early 20s.

The lowest net undercount rate was for people aged 55 years and over (-0.1%).

In general, males had a higher net undercount rate (2.2%) than females (1.2%).

2 NET UNDERCOUNT(a), Sex by age group - 2011

Males

Females

Persons

Age

no.

SE

no.

SE

no.

SE

0-4 yrs

8 488

5 925

8 775

5 114

17 263

8 017

5-9 yrs

13 559

5 914

6 367

5 045

19 926

7 781

10-14 yrs

4 465

5 124

1 449

5 538

5 914

7 664

15-19 yrs

18 005

6 396

17 890

6 577

35 895

9 044

20-24 yrs

62 594

9 392

45 555

7 730

108 148

12 812

25-29 yrs

61 035

10 613

31 602

7 647

92 636

13 444

30-34 yrs

30 419

7 225

14 527

6 528

44 946

10 407

35-39 yrs

15 746

7 330

1 462

5 909

17 207

9 424

40-44 yrs

8 768

6 307

12 255

6 077

21 023

8 856

45-49 yrs

11 367

6 141

-5 876

5 448

5 491

8 063

50-54 yrs

11 848

6 279

1 900

5 428

13 747

8 411

55 yrs and over

-5 498

7 396

-2 161

7 626

-7 659

11 500

Total all ages

240 796

24 534

133 744

20 772

374 540

38 315

(a) A negative value indicates a net overcount.

3 NET UNDERCOUNT RATE(a), Sex by age group - 2011

Males

Females

Persons

Age

%

SE

%

SE

%

SE

0-4 yrs

1.2

0.8

1.3

0.7

1.2

0.6

5-9 yrs

1.9

0.8

1.0

0.8

1.5

0.6

10-14 yrs

0.6

0.7

0.2

0.8

0.4

0.6

15-19 yrs

2.4

0.8

2.6

0.9

2.5

0.6

20-24 yrs

7.8

1.1

6.0

1.0

6.9

0.8

25-29 yrs

7.5

1.2

4.0

0.9

5.8

0.8

30-34 yrs

4.1

0.9

1.9

0.9

3.0

0.7

35-39 yrs

2.1

0.9

0.2

0.8

1.1

0.6

40-44 yrs

1.2

0.8

1.5

0.8

1.3

0.6

45-49 yrs

1.5

0.8

-0.8

0.7

0.4

0.5

50-54 yrs

1.6

0.9

0.3

0.7

0.9

0.6

55 yrs and over

-0.2

0.3

-0.1

0.3

-0.1

0.2

Total all ages

2.2

0.2

1.2

0.2

1.7

0.2

(a) A negative value indicates a net overcount.

4 NET UNDERCOUNT RATE, Sex by age group - 2011

STATES AND TERRITORIES

The challenges facing Census enumeration vary between states and territories. Table 5 shows the rates of net undercount for Australian states and territories for Censuses from 1991 to 2011.

As in previous Censuses, in 2011 the Northern Territory recorded the highest net undercount rate of all states and territories (6.9%), while the Australian Capital Territory continued to record the lowest net undercount rate (0.7%).

While the two territories reflected the minimum and maximum net undercount rates, Victoria and South Australia continued to show relatively low rates (both 1.1%). Western Australia had the highest rate for a state (2.5%), emphasising the continued coverage challenges in that state. All states and territories had a lower net undercount rate in 2011 than in 2006, except for Tasmania which was relatively consistent at 2.0%. The greatest decreases were for Queensland (3.7% in 2006 to 1.8% in 2011) and Victoria (2.3% in 2006 to 1.1% in 2011).

It is important to note the effect of the introduction of Automated Data Linking (ADL) when considering the changes from 2006. For more information, see the Statistical Impact of ADL Technical Note (in Explanatory Notes).

5 NET UNDERCOUNT, State/territory of usual residence - 1991-2011

1991

1996

2001

2006(a)

2011(b)

Persons

Rate

Persons

Rate

Persons

Rate

Persons

Rate

Persons

Rate

no.

%

SE

no.

%

SE

no.

%

SE

no.

%

SE

no.

%

SE

New South Wales

109 200

1.9

0.1

91 400

1.5

0.2

130 106

2.0

0.2

157 578

2.4

0.4

136 647

1.9

0.4

Victoria

78 800

1.8

0.1

74 000

1.6

0.3

67 254

1.4

0.2

113 596

2.3

0.4

56 906

1.1

0.3

Queensland

52 100

1.8

0.1

57 300

1.7

0.3

68 514

1.9

0.2

148 409

3.7

0.4

77 215

1.8

0.4

South Australia

22 300

1.6

0.1

19 300

1.3

0.3

24 293

1.6

0.2

36 281

2.3

0.4

17 283

1.1

0.4

Western Australia

33 200

2.1

0.2

28 100

1.6

0.3

37 446

2.0

0.3

64 150

3.2

0.6

57 918

2.5

0.5

Tasmania

7 700

1.7

0.2

6 600

1.4

0.4

7 410

1.6

0.3

9 535

2.0

0.6

10 261

2.0

0.6

Northern Territory

4 800

2.9

0.7

5 700

3.1

1.6

7 814

4.0

0.6

15 909

7.6

1.5

15 716

6.9

1.3

Australian Capital Territory

4 100

1.4

0.2

3 400

1.1

0.3

3 282

1.0

0.4

4 027

1.2

1.0

2 595

0.7

0.8

Australia

312 300

1.8

0.1

285 800

1.6

0.1

346 119

1.8

0.1

549 486

2.7

0.2

374 540

1.7

0.2

(a) Care should be taken when comparing estimates from 2006 onwards with previous years due to changes made to PES estimation and the inclusion of remote areas and discrete Indigenous communities in the PES sample from 2006.

(b) Care should be taken when comparing 2011 estimates with previous years due to changes made to PES linking and matching methodology. For more information see Linking and Matching.

GREATER CAPITAL CITY/REST OF STATE REGION

The regional differences in net undercount in the 2011 Census for greater capital cities and the rest of state regions are presented in tables 6 and 7.

Greater capital cities are represented by Greater Capital City Statistical Areas (GCCSAs) and represent a socio-economic definition of each of the eight state and territory capital cities. This means each greater capital city includes people who regularly socialise, shop or work within the city but live in small towns and rural areas surrounding the city. 'Rest of state' regions are the areas within each state or territory not defined as being part of the greater capital city.

Different problems are encountered in enumerating different areas of Australia and these are reflected, to a certain extent, in the net undercount rates. In urban areas, locating dwellings is generally easier but contacting occupants and gaining their cooperation can be more difficult. In contrast, in rural and remote areas where dwellings may be scattered over a wider area, locating the dwellings can cause considerable difficulties. In 2011, New South Wales, Tasmania and the Northern Territory had higher net undercount rates in their rest of state regions compared with their greater capital cities. All other states and territories had a lower net undercount in their rest of state regions (compared to their greater capital cities).

At the broad Australia level, the total net undercount rates in 2011 were slightly lower for the rest of state regions (1.7%) compared to greater capital cities (1.8%). This contrasts with 2006, when the net undercount rate for balance of state/territory (3.0%) was higher compared to the capital cities (2.5%). As was the case in 2006, in 2011 the Northern Territory showed the largest difference in net undercount rate between its greater capital city and rest of state region (3.7% and 10.9% respectively).

It is important to note that for the 2011 PES, the Australian Statistical Geographical Standard (ASGS) replaced the Australian Standard Geography Classification (ASGC) as the framework for PES geography. The move to the new geography will allow for improvements in the quality of small area time series data from the Census. However, the change has resulted in an unavoidable break in series and care should be taken when comparing the 2011 greater capital city/rest of state net undercount estimates to the 2006 capital city/balance of state net undercount estimates.

Table 8 shows net undercount estimates and rates by registered marital status by sex. The net undercount rates were highest for people identified as never married (3.7%) and lowest for people widowed, divorced or separated (a net overcount of 0.8%). It is important to consider the strong relationship with age when interpreting net undercount estimates by registered marital status.

8 NET UNDERCOUNT(a), Registered marital status by sex - 2011

Males

Females

Persons

Persons

Rate

Persons

Rate

Persons

Rate

no.

SE

%

SE

no.

SE

%

SE

no.

SE

%

SE

Never married(b)

234 038

20 842

4.2

0.4

151 557

16 596

3.1

0.3

385 595

29 990

3.7

0.3

Widowed, divorced or separated

-8 837

10 274

-0.8

1.0

-15 434

9 878

-0.8

0.5

-24 271

14 630

-0.8

0.5

Married

15 594

10 970

0.4

0.3

-2 378

9 966

-0.1

0.2

13 216

18 737

0.2

0.2

Total persons

240 796

24 534

2.2

0.2

133 744

20 772

1.2

0.2

374 540

38 315

1.7

0.2

(a) A negative value indicates a net overcount.

(b) Includes those who are living with a de facto partner and have never been in a registered marriage.

INDIGENOUS STATUS

Special procedures are used in the Census to support the enumeration of the Aboriginal and Torres Strait Islander population, as counting this population continues to present a number of challenges.

The 2011 PES estimated that 662,335 Aboriginal and Torres Strait Islander persons should have been counted in the Census, compared with 513,977 persons in 2006.

Table 9 shows net undercount estimates by Indigenous status. In 2011, the net undercount rate was 17.2%, compared with 11.5% in 2006. The net undercount for 2011 was estimated to be 114,188 persons, which was almost double the 2006 estimate of 59,178 persons.

9 NET UNDERCOUNT, Indigenous status - 2011

Persons

Rate

no.

SE

%

SE

Indigenous

114 188

14 274

17.2

1.8

Non-Indigenous

1 318 799

37 272

6.2

0.2

It is important to note that these measures refer to the undercount of persons according to their Indigenous status, regardless of whether or not they were actually counted in the Census. In other words, persons who were counted in the Census and had a 'not-stated' Indigenous status will not be included in the Census counts of either Aboriginal and Torres Strait Islander or non-Indigenous persons, but are, instead, a separate category for this classification. They will, however, be included in Census counts for other key categories, such as Age and Sex.

In order to understand the differences between the 2006 and 2011 PES results, the ABS undertook an extensive quality assurance process. The results of this process are summarised in the Improvement in collection of Indigenous status Technical Note (in Explanatory Notes).

This quality assurance process has led the ABS to advise caution when comparing net undercount for Aboriginal and Torres Strait Islander persons between 2006 and 2011. Analysis of data indicates that the main contributing factor for the difference between the 2006 and 2011 estimates was improved PES methodology and procedures, which resulted in better identification of Aboriginal and Torres Strait Islander people in the 2011 PES.

Historically, the ABS has a program of continuous improvement in its survey methodologies. Improvements to the PES in 2006 and 2011 are summarised in Survey Enumeration, Linking and Matching and Estimation. While the individual impacts of all improvements made in 2011 cannot be measured, they have resulted in a change in the Indigenous status classification of Aboriginal and Torres Strait Islander persons in the PES and the Census between 2006 and 2011. This has in turn resulted in a noticeably different net category change for Indigenous status in net undercount estimates, and accounted for most of the change in the estimate of net undercount for Aboriginal and Torres Strait Islander persons between 2006 and 2011.

It is also important to note that Indigenous status, as collected in both the Census and PES, is based on responses to a question related to information that some people will consider personal and sensitive. Respondents can choose to indicate in the Census that they are Indigenous or non-Indigenous, or they can choose to not answer the question at all. If no answer is provided, the Census does not impute for this missing response (which is also the case for imputed persons). The Census count is therefore a count of those who were identified by a respondent as Indigenous (i.e. those without a response are excluded).

COUNTRY OF BIRTH

As Census forms are generally completed by one or more persons in a household, those who have come to Australia from other countries and whose first language is not English may find completing a Census form more difficult than other Australians. For several Censuses, special strategies have been employed to promote an understanding of the Census among migrants, in particular that the Government is not using their information for anything other than statistical purposes and to provide assistance in a range of languages.

Tables 10 and 11 show the undercount estimates and rates by country of birth. The countries displayed were the 10 highest ranked (in terms of population residing in Australia) according to the 2011 Census. There were 1,195,432 people (5.6% of the Census count) whose country of birth was not stated in the Census. Since Census does not impute a Country of birth for these people, the PES estimates of net undercount are not adjusted to take account of any imputed persons. As with Indigenous status, these people, while counted in the Census, do not contribute to the Census counts for these categories but do count to PES estimates of their population. For further information about Census not-stated responses and their impact on estimates of net undercount see Components of net undercount.

Of those countries listed, persons born in China had the highest net undercount (55,965 persons) followed by New Zealand (46,536 persons). China also had the highest net undercount rate (14.9%) followed by India (9.7%). Persons born in Scotland had the lowest net undercount rate (1.2%) followed by those born in England and Italy (both 4.6%).

10 NET UNDERCOUNT(a)(b), Country of birth by sex - 2011

Males

Females

Persons

Country of Birth

no.

SE

no.

SE

no.

SE

Australia

584 461

19 579

477 937

16 850

1 062 398

30 156

England

26 257

4 870

17 938

4 549

44 195

7 049

New Zealand

30 504

4 323

16 032

3 770

46 536

6 344

China

28 239

5 375

27 726

5 033

55 965

8 934

India

22 309

4 371

9 386

2 856

31 694

6 075

Italy

4 880

1 631

4 004

1 654

8 884

2 513

Vietnam

8 020

2 736

8 075

2 326

16 095

4 313

Philippines

6 441

1 952

10 665

2 417

17 106

3 603

South Africa

7 099

2 138

4 613

1 791

11 712

3 383

Scotland

-1 348

1 511

2 929

1 674

1 582

2 208

Other overseas

151 369

11 502

122 435

8 990

273 804

15 894

(a) Net undercount is based on Census counts for a category. In the Census, Country of birth was set to not-stated where the response was blank and where imputed person records were created for non-responding dwellings. Hence net undercount estimates for Country of birth do not sum to the Australia total.

(b) A negative value indicates a net overcount.

11 NET UNDERCOUNT RATE(a)(b), Country of birth by sex - 2011

Males

Females

Persons

Country of Birth

%

SE

%

SE

%

SE

Australia

7.3

0.2

5.9

0.2

6.6

0.2

England

5.4

1.0

3.8

0.9

4.6

0.7

New Zealand

11.1

1.4

6.3

1.4

8.8

1.1

China

16.6

2.6

13.5

2.1

14.9

2.0

India

12.0

2.1

6.7

1.9

9.7

1.7

Italy

4.9

1.6

4.2

1.7

4.6

1.2

Vietnam

8.6

2.7

7.5

2.0

8.0

2.0

Philippines

9.1

2.5

9.1

1.9

9.1

1.7

South Africa

9.0

2.5

5.9

2.2

7.4

2.0

Scotland

-2.1

2.4

4.2

2.3

1.2

1.6

Other overseas

11.2

0.8

8.8

0.6

10.0

0.5

(a) Net undercount is based on Census counts for a category. In the Census, Country of birth was set to not-stated where the response was blank and where imputed person records were created for non-responding dwellings.